January Jones reportedly dropped two dress sizes for her role in X Men: First Class.

The Mad Men actress went on a strict 'juice diet' to take her from a UK size 8 to a 4, according to Now.

Speaking about the costume for her role as Emma Frost in X Men, Jones said: 'I'm virtually naked. In the comic, it's pretty much painted on and she has quite an unrealistic body. It was a challenge for me."

A source described Jones as "a perfectionist" and said that the actress "prefers herself pin thin".

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"She went on a strict juice fast," the source said. "She also ate tons of raw veg and every third day allowed herself some lean meat.

"It was extreme, but she needed that boost of confidence she gets from being thin. The truth is she's insecure. Like a lot of people, she has a distorted view of herself."